In one of my previous incarnations I was an immigration officer in the UK. Sanctum and pharvey are posting at opposite ends of the spectrum, but both are right.
In no particular order:
1. Many officials of all varieties suffer from the "Little Hitler" syndrome.
2. Politeness works both ways. It just so happens that my personal approach was (usually) friendly, but that was just my style. In the other direction, it was always interesting to see which 'celebrities' took a friendly approach and who played the "Don't you know who I am?" card. (Never a good idea...

)
3. Some punters wouldn't appreciate a happy/smiley/jokey immigration official after they'd been standing in a Heathrow queue for 2 hours...
Some of my favourite celebs:
Sterling Moss
Barry Sheene
Paddy Ashdown
And the worst ever:
Hughie Green
Some random stories:
I was once on the British desk at Dover. Long queue. Harassed woman arrives carrying luggage in both hands and with 2 passports between her teeth. She mumbles an apology as she leans towards towards me. "You shouldn't talk with your mouth full, Madam" says the oh-so-funny immigration officer. "Don't worry, mate, she never does..." says her husband just behind her.
In the late 70s EU citizens were still sort of subject to control as they entered the UK. At Dover we got so bored we used to play silly games when we thought we could get away with it. Ferries from Zeebrugge would have 100s of cars carrying Dutch families to Camber Sands.
IO: "How many cats are spayed in the UK?"
Dutch tourist: "2 weeks"
etc.
One colleague got caught out as a middle aged Japanese guy pulled up at the car kiosk. When he asked how long the man would stay, the Jap just stared blankly. Cue silly humour:
IO: "How rong you stay Engran?"
Jap guy: "Oh sorry, didn't quite hear you the first time. Probably just a couple of weeks. I have a series of meetings with the governor of the Bank of England..."
And there was the story of Peter Cook and Dudley Moore sauntering up to a deserted British desk at Heathrow after all the other passengers had gone through. The IO put down his newspaper and took his feet off the desk. Peter Cook stood there while Dud bent down to get his passort out of his bag. The IO spontaneously said, "Come on, c#nt" Dud looked up and said "Don't call me a c#nt, you f####ing c#nt" etc.
It would be
so good if that story was true.
Of course all that was a very long time ago...
Sorry for the brief diversion - back to topic.